1. Field of the Invention
This invention relates to systems and methods for wireless and wired optical data communications, in particular infrared data communications using techniques borrowed ultra wideband (UWB) radio.
2. Background Art
Standards for infrared data communication are defined by the Infrared Data Association (IrDA: www.irda.org) and cover data communication rates to around 4 Mbps. The standards are generally directed to point-to-point connectivity at distances of up to 1 m. Using transceivers that broadcast infrared pulses in a cone of up to 30 degrees half angle off centre. The specifications also require that a maximum irradiance is not exceeded so as not to swamp and “blind” a receiver, for similar reasons the transmissions are half-duplex. The IrDA protocol stack bundles data for transmission into a data packets with start and stop bits, and, optionally, a CRC value. Over this runs a Link Access Protocol (LAP) for device discovery and establishing a “virtual wire” connection, a Link Management Protocol (LMP) which allows one or more services to be registered and operate over a connection; other, higher level protocols may also be implemented. There is interest in operating IrDA links at higher data rates but there are problems with the IrDA protocols when the data rates are increased significantly. There are also general problems with infrared (and optical) data communications including interference, for example from fluorescent lights, and multipath dispersion, which can cause inter symbol interference (ISI). Some background prior art relating to optical and infrared communications can be found in D Shiu and J M Kahn; “Differential Pulse-position Modulation for Power-efficient Optical Communication”, IEEE Trans. Comm. 47:1201-1210 (1999), and in U.S. Pat. No. 6,414,774.
The inventors have recognised that techniques borrowed from ultra wide band (UWB) radio communications may advantageously be employed to provide improved optical communications systems. Ultra wide band communications systems typically employ very short pulses of electromagnetic radiation resulting in a spectrum with a very wide bandwidth; when an antenna is substantially directly excited with such a pulse the system may be referred to as carrierless or carrier free since the resulting rf emission lacks any well defined carrier frequency. Other UWB techniques employ multiband OFDM (orthogonal frequency division multiplexed) technology. The techniques used in UWB receivers, combined with the very short pulses employed, make radio frequency UWB receivers relatively robust with respect to multipath effects. The inventors have recognised that similar techniques may be employed in the optical field in order to mitigate optical ISI. Furthermore some particular techniques employed by the inventors in their UWB receiver systems may also be used to implement new types of optical communication system.